During the past years, Nd:YAG welding lasers have found increasing applications in the areas of jewelry and dentistry. These so-called handheld welding lasers are suitable to perform precise point and seam welding in the sub-millimeter region. They have not only the advantage of providing solder-free joints, but also protect the workpiece as compared to the conventional flame welding technique. A typical construction of devices presently on the market is illustrated in FIG. 1. The conventional xe2x80x9cclassicxe2x80x9d resonators are mostly constructed in the manner illustrated in FIG. 2a and have a planar output mirror 2 and a concave reflecting mirror 3. The Nd:YAG laser rod 1 is located approximately in the center between the mirrors 2 and 3.
The exiting laser beam 4 is focused onto the work plane 8 by a beam expander 5 with an adjustable divergence, a turning mirror 6 (for example, 1064 nmxe2x80x94HR, visible rangexe2x80x94AR) and a focusing lens 7.
All devices encounter problems associated with xe2x80x9cthermal lensingxe2x80x9d of the Nd:YAG rod and the xe2x80x9cinitial pulse characteristicsxe2x80x9d associated therewith. Pumping with a flash lamp and water cooling produces a radial temperature profile in the Nd:YAG rod, which is transformed by the characteristic material constant dn/dT into a refractive index profile and thereby into a lensing effect. Depending on the injected pump energy and the cooling provided by the cooling water, respectively, this lensing effect depends on the pump power. FIG. 3 illustrates the initial pulse characteristics for a state-of-the-art xe2x80x9cclassic resonatorxe2x80x9d, wherein the spot size at the focal point is simulated without beam expansion as a function of the pump power, i.e. with varying thermal lensing effect and for different radii of the reflecting mirror. The increase of the focal diameter with increasing pump power is clearly seen. Smaller radii of curvature of the reflecting mirror lead to a smaller relative change, but to overall larger values of the focal diameter. For single pulses or for initial pulses (low pump power), the lensing effect is still small. For continuous pulses (high pump power) and a predetermined frequency the lens increases to a value which depends on the average pump power in continuous operation. This lensing effect affects the beam quality and thereby also the spot size in the work plane as well as (to a lesser degree) the pulse energy. The user who is mainly interested in the energy density, i.e. the pulse energy divided by the spot size, will recognize this phenomenon as a strongly variable welding outcome which depends on the welding history.
One possibility to avoid this problem is to transmit the laser beam through a sufficiently long glass fiber. Since the glass fiber does not preserve the diameter of the beam, the beam which is coupled out typically has a constant diameter and an approximately constant divergence. However, this approach degrades the beam quality, so that the focusing unit has to be adapted accordingly. Moreover, the so-called xe2x80x9cbenign behaviorxe2x80x9d of the welding process suffers, since the depth of focus in the work plane is reduced. Another possibility to avoid this problem is to use a stronger beam expansion before the beam splitter and to also work outside the focusing range, where the image of the rod surface remains approximately constant. This approach also reduces the initial pulse characteristics. However, the xe2x80x9cbenign behaviorxe2x80x9d is again adversely affected (depth of focus of the laser focusing system in the work plane).
The publication by MAGNI, V. et al.: xe2x80x9cRecent Developments In Laser Resonator Designxe2x80x9d in Optical and Quantum Electronics 23, 1991,pp. 1105-1134, in particular page 1106,second paragraph, describes additional measures to counteract or even compensate the effect of thermal lensing. These conventional measures, however, are only effective at a specified value of the pump power.
It is an object of the invention to provide a stable resonator which is stable over an extended range of the pump power against the effects caused by thermal lensing, rather than only at a specific value of the pump power. The resonator according to the invention should also reduce the initial pulse characteristics below the detection limit of the user, while at the same time maintaining the xe2x80x9cbenign behaviorxe2x80x9d, i.e. the depth of focus, of the laser.
The solution of this object is provided by resonators as described in the commensurate claims 1, 3, 4 and 5. The applicant has realized that, unlike in state-of-the-art devices, the beam quality as a function of the pump power has a comparably flat maximum for relatively short resonator lengths due to the extremely asymmetric construction of the resonators according to the invention. Accordingly, the applicant achieves a comparably constant beam quality over a larger pump power range. As a result, the thermal lensing effect has no effect or only an insignificant effect on the welding result; the characteristics features of the initial pulse are negligibly small. While the laser rod in the embodiments recited in the commensurate claims 1 and 4 is completely displaced towards the output side, in the other advantageous embodiments recited in claims 3 and 5 the laser rod may be located at a very short distance from the output mirror.